1. Field of the Invention
The present invention relates to a fluoropolymer tape possessing extraordinary abrasion resistance, good flexibility (without breakage), excellent dielectric properties, and which when laser marked demonstrates durable markings. More particularly, the present invention is directed to compositions of unsintered polytetraflouroethylene (PTFE) containing virgin, non-thermally cycled, polyphenylene sulfide milled to a particle size of less than about twenty-five microns. In a preferred embodiment the PTFE/polyphenylene sulfide mixture also contains a laser markable pigment, preferably a metal oxide. In a particularly useful PTFE tape, the PTFE composition comprises in addition a oxybenzoyl homopolyester (poly-p-oxybenzoate) which lends significantly improved abrasion resistance.
2. Background of the Related Art
Polytetrafluoroethylene (PTFE) tape is used in many applications including sealing joints, insulating conductive wires, and protecting materials from corrosive elements. PTFE demonstrates good chemical and heat resistance, and electrical insulation characteristics, as well as a low coefficient of friction. However, in general, it has less than desirable mechanical properties in particular with respect to abrasion resistance and compression strength.
There is a considerable need in the aerospace industry for wire and cable insulation that have a low dielectric constant, that are resistant to chemicals and solvents, can withstand large temperature variations, are light-weight and fire retardant, produce low smoke and fumes on combustion/melting, are easily stripped from the conductor, and that are generally safe for use. It is also desired that aerospace insulating materials take up as little room as possible, be flexible and be able to withstand considerable bending stresses.
Many of the insulation materials used in the aerospace industry comprise hexafluoropropylene, perfluoropropylene, or perfluorovinyl ethers. While these insulation materials have good chemical resistance to fluids with which they may come into contact, and possess good dielectric and weather properties, as well as many of the other properties sought out by the aerospace industry, they suffer from less than desirable mechanical properties in that they offer less than desired resistance to scrape abrasion and exhibit less than desirable resistance at temperatures above 180° C. (which may be encountered in avionic systems).
Given the many significant advantages proffered by fluoropolymers as insulators, numerous proposals have been suggested and adopted by the aerospace industry to address the mechanical deficiencies of the compounds.
One commonly used approach is to make composites which incorporate polyimide resins along with the fluoropolymers. Typically such insulation is based on polyimide films such as Kapton® which are coated or laminated over with tetrafluoroethylene polymers. The films are slit into tapes which may be taped over, or extruded over, the fluoropolymers. Fluoropolymers, such as polytetrafluoroethylene (“PTFE”), may also be co-extruded with fillers such as polyimide resins, polyamide-imide resins and polyamide resins and molded onto the wire or cable (See, e.g. EP 0 023 047; See, also, Japanese Patent Application No. 61-31448 which discloses a wear resistant polytetrafluoroethylene composition comprising polytetrafluoroethylene and a polyimideimidazopyrrolon resin).
Porous PTFE filled with abrasion resistant filler such as graphite has also been asserted to improve the longevity of PTFE wire/cable coverings (See, e.g., U.S. Pat. No. 5,636,551). The fluoropolymer may also be protected from abrasion by coating with a closed-cell polymer having better abrasion resistance (See, e.g., U.S. Pat. No. 5,210,377).
Japanese Patent Application No. 63-118357 discloses a tetrafluoroethylene resin composition comprising 50 to 90 weight percent tetrafluoroethylene resin with 10–50 weight percent fine powder of polyether ether ketone resin having a particle diameter of between 1 and 50 μm. The material is said to have excellent compression creep characteristics as well as sealing performance without impairing the low friction characteristics of the tetrafluoroethylene resin.
Several laminates and compositions are known to arc track under certain environmental conditions. Arc tracking is a catastrophic failure caused by an electrical arc when a short circuit occurs between the conductor and a conducting medium external to the insulation, such as a moderately conductive fluid. Arc tracking is a particular potential problem with polyimide-fluoropolymer insulations which employ TFE copolymers to bond layers. Use of PTFE/TFE copolymers as the binding agent between layers, as disclosed, for example, by U.S. Pat. No. 5,106,673, while somewhat diminishing arc tracking, has not been found to be entirely satisfactory.
Wire and cable in the aerospace industry also needs to be conspicuously labeled in order to permit easy replacement. Labeling of wire/cable in the aerospace industry is particularly important given the safety considerations involved, as well as the commonplace need for expeditious servicing of equipment. Labeling on wire and cables must be able to withstand the many environments to which the wire/cable may be exposed, and must remain with the wire/cable for its serviceable life. It is a general practice in the aerospace industry to mark individual electrical cables repeatedly along their length with identification numbers.
As fluoropolymers tend to be non-tacky, printing on such materials with conventional inks is difficult, and often less than permanent. Polyimides also suffer from less then desirable print fastness.
In order to improve fastness of inks, it has been proposed that the surface of the fluorine resin be modified by mix the fluorine resin (including PTFE) with a light-absorbing material (such as whole aromatic polyester, poly (ether ether ketone), polyamide, poly(ether ketone), poly(phenylene sulfide), aromatic polyamide, polyarylate, poly(ether imide), poly(amide imide), polysulfone, poly(ether sulfone), a metal oxide (such as zinc oxide, zirconia or titanium oxide), and metal sulfides (such as molybdenum disulfide)) and then irradiating laser light on the surface of the composite (See, U.S. Pat. No. 5,320,789). Such treatment is said to improve adhesion and wetting properties of numerous fluorine resins.
Hot stamp printing, while generally more permanent than ink printing, has the disadvantage that it degrades the thermal insulating properties of the cable which can cause arc tracking. Press-marking while somewhat more permanent than inks suffers in that the insulation is stretched at the press points, making the insulation more liable to peeling and breaking.
Marking of items using laser light has been known for some time (See, e.g., EP Patent Application No. 0 249 082 which teaches laser marking the keys of keyboards comprising a polycarbonate containing 10 to 50% of an aromatic polyester (which may be a condensate made from bisphenol-A terephthalic acid and butylene glycol)). It is therefore not surprising that a number of techniques have been developed to print marks onto the insulation of wires and cables using laser light.
One technique which has been employed entails coating the wire/cable with colored emulsions which change color when irradiated by a laser or that include an outer layer which is colored differently from an underlying area such that when the laser removes a portion of the outer layer the underlying differently colored underlayer becomes visible. For example, EP Patent Application No. 0 447 032 discloses a laser markable white pigment fluoropolymer (which includes PTFE) composition which includes a first pigment which is markable by ultraviolet laser and a second pigment which is non-absorbing in the ultraviolet region of the optical spectrum and which has a white appearance in the visible region of the optical spectrum. The first pigment may be antimony trioxide, titanium dioxide, polyethylethylketone (PEEK) and/or polyethylsulfone (PES), while the second pigment may be silicon dioxide, magnesium oxide, aluminum oxide and diamond. A preferred composition comprises from 1 to 35% by dry weight titanium dioxide and from 2 to 30% by dry weight second pigment.
U.S. Pat. No. 5,223,358 to Yamada et al. teaches laser marking (with light having a wavelength of 600 nm or less, thus including the ultraviolet range) a substrate coated with a fluororesin composition (including PTFE) comprising a high-molecular weight material having a benzene ring and at least one of a nitrogen atom, a sulfur atom and a carbonyl group in the main chain thereof (such as a polyamideimide, polyimides, polyparabanic acid, polyether imides, sulfone polymers such a poly sulfone, polyether sulfones and polyaryl sulfones, polyphenylene sulfide, polyether ether ketones, and polyoxybenzoyl). The laser light is said to cause a change in the color of the high-molecular weight material consistent with the irradiated surfaces (providing a difference in color between irradiated parts and unirradiated parts). Pigments such as titanium oxide in an amount of 0.1 to 20 parts by weight, or mica or pigment-coated mica in an amount of from 0.1 to 5 parts by weight per 100 parts by weight of the fluororesin composition may be added to the fluororesin composition.
It also known to non-aggressively mark fluoropolymer insulation that contains a photosensitive material such as titanium dioxide, zinc dioxide or tin dioxide (See, e.g., GB 2,215,116).
French Patent No. 2,732,030 discloses a PTFE insulation comprising uncured PTFE having 0.1 to 5.0% by weight titanium oxide pigments and 0.1 to 5.0% of an organic polymer chosen from the group comprising arylene sulfide polymers, in particular, polyphenylenesulfide, polyarylsulfones, polysulfone, polyethersulfone, and polyaryletherketones such as polyetherketone and polyetheretherketone. Such insulation material is said to be able to be marked with an ultraviolet laser to lend marking contrast reaching and exceeding 80%.
In U.S. Pat. No. 5,501,827, assigned to the assignee of the present application, a laser-markable composite material for application to wires and cables, and a process for fabricating the same, is disclosed. Such marked composition is taught to retain good color contrast after heat aging. The composite material comprises a polytetrafluoroethylene resin and a photosensitive filler material (e.g. titanium dioxide) along with an extrusion aid, the polytetrafluoroethylene resin being air-milled with the photosensitive filler material to a uniform dispersion, mixed with an extrusion aid, and then paste extruded to produce an object, direct coating, a tape, ribbon, etc. Such material is asserted to be more stable than heat aging than the prior art, with the disclosed material asserted to lose only about 10% of its contrast upon accelerated heat aging as compared to the prior art which lost nearly 50%.
While the prior art suggests different compositions and methods for improving the hardness of PTFE-based insulation and improving the laser markability of such materials, and while PTFE-based insulation on the whole are believed to have good chemical resistance and fire retardance, it is generally desired in the art that presently available PTFE-based insulation be improved with respect to dielectric behavior, temperature and weather resistance, marking stability, and with respect to the possession of both toughness and flexibility properties.